Carrier frequency modulation



March 10, 1942.

H. N. KOZANOWSKI ETAL CARRIER FREQUENCY MODULATION Filed 001;. 27, 1938 2 Sheets-Sheet 1 Oscillator 35 m j g3? flmplzfl' 6 115 llodu Z a i- [an l requer rc y flm lz'fler I WITNESSES:

INVENTORS Henry AZ K'ozanows/rz' and I Zia E. Nauromtseff.

ATTORN.

0 F v Car/"zr Tube 2 V March 10, 1942. H. N. KOZANOWSKI ETAL 2,275,970

CARRIER FREQUENCY MODULATION Filed Oct. 27, 1938 2 Sheets-Sheet 2 A A V V V V V V k WW F Dual OUZput k Mloloao 130P/mse EeZazz'on, RF." I WITNESSES: I INVENTORS f"! N lfozzznowskz and M A lZza fifzou/ orfltseffi Patented Mar. 10, 1942 CARRIER FREQUENOY'MODULATION Henry N. Kozanowski, Collingswood, and Ilia E. Mouromtsefl, Montclair, N. J., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 27, 1938, Serial No. 237,262

Glaims.

This invention relates to carrier-wave transmission and more particularly to a method and means for modulating the carrier-frequency energy to be transmitted.

At the present state of the art, two distinct types of modulation are generally known. One is by varying the fundamental carrier-frequency in accordance with a modulation or audio-frequency in which case the modulation is referred to as frequency modulation; or by the second, commercially more accepted method wherein the carrier-frequency remains constant and its amplitude is made to change in accordance with the modulation-frequency. In the latter method to which this invention is directed, the modulated radio-frequency carrier can be considered. as made up of three radio-frequency components combined in such manner that their addition gives the conventional modulation wave. In customary class C modulated amplifiers the carrier output is caused to increase and decrease in amplitude at modulation-frequencies by variation of the anode voltage of the amplifier due to the action of the modulator. Thus the direct-current anode voltage for 100% modulation swings up to twice carrier value and down to zero voltage at a modulation frequency addition and subtraction from carrier-amplitude.

In the various methods heretofore known to accomplish amplitude-modulation, certain disadvantages resulted from the fact that the instan- Another object of this invention is to provide a system of modulated carrier-frequency transmission wherein circuit components may be reduced in number and considerably in size thereby effecting a substantial saving in the cost of the equipment.

Other objects and advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims, and taken in connection with the accompanying drawings in which:

Figure 1 shows schematically the modulation system in accordance with this invention, and

Fig. 2 shows a series of curves illustrating the process of modulation.

Briefly, the invention consists in the arrangement of a circuit in which three tube units forming a plurality of transmission channels for carrier-frequency energy are so related that one supplies carrier power only, the second adds to carrier, and the third subtracts from the carrier in order to produce a final modulated radiofrequency output wave which is a faithful reproduction of the modulated-frequency sign-a1 applied to two of said channels in order to control the energy transfer therein.

Referring to the drawings, vacuum tubes I, I' and 2 have their control grids 3, 4 and 3 respectively connected jointly to a common impedance represented by the inductance 5 which, in turn, is excited through capacitive coupling by means of condenser 6 by a source of radio-frequency energy represented here by a conventional square marked oscillator and amplifier. The type of energy producing means has no particular bearing on the modulation system herein described, and can be of any conventional type. In practice, 'a crystal controlled master oscillator may be employed followed by a number of high frequency amplifying stages, the output of which having the required radio-frequency power may be coupled through coupling condenser B to simultaneously excite the grid circuit of tubes I,

I' and 2. The grid return of the tubes is effected through the inductance 5 and filter resistor I, Icy-passed by condenser 6 and is shown to terminate at the negative side of a suitable direct current :power supply source, schematically shown here by the filter choke 8, filter condensers 9 and III and the voltage divider resistor II.

The cathodes I2, I 3 and I2 of the tubes termiminal of the primary winding is by-passed to ground by condenser 2I.

The anode II of the tube I is similarly connected to a tank circuit, the components of which.

are indicated by identical reference characters bearing primary indices. The high potential terminal of both tank circuits, that is, the anode connection of the tubes I1 and I I includes a branch circuit comprising radio frequency chokes 22 and 22', conductors 23 and 23' which terminate in the secondary winding 24 of a modulation-frequency or audio-transformer 25. The

secondary winding thereof is center-tapped to" ground, and its primary winding 26 excited from a modulation-frequency amplifier represented here by a conventional square similarly marked. The input to the be a microphone 21.

In the transmission of intelligence by voice the modulation-frequency is, of course, within the audio range and may be termed the audio-freapplied by energizing the microphone 21, one

terminal of the secondary winding 24 of the audio-frequency transformer becomes positive with respect to the grounded center tap in a manner conforming with the character of the audio signal. The audio voltage developed between ground and the particular positive terminal of the secondary winding 24 at that instant becomes the anode voltage for one of the tubes I or I. The particular tube then will act as an amplifier and deliver a. radio-frequency output into the load circuit represented by the amplifier is shown to' ing in amplitude at audio-frequency rate. Durprimary 35 of the transformer 36 which is varying this positive voltage interval the tube and circuit "attachedto the other terminal of the secondary winding 24 which at this instant is at quency employed for modulation. Needless to The anode 28 of the tube 2 connects also to u a tank circuit comprising radio-frequency transformer 29, primary winding 30, and condenser 3|. The anode return of the primary 30 is connected to the positive terminal of the direct current power supply by means of conductor 32. The secondary windings 33, 34 and 33' 0f the tank circuit transformers I9, 29 and I9, respectively, are connected in a series circuit together with the primary winding 35 of the output transformer 35, the secondary winding 31 of which connects to the antenna 38 and to ground;

The various heater circuits for the tubes have been omitted in order to simplify the schematic showing. The tubes shown are of the indirectly heated cathode type triodes the filaments of which (not indicated here) may be connected together to any suitable source of alternating or direct current. Those skilled in the art are well aware of the fact that such tubes operate only when their cathodes are properly heated, and, the omission of the heating circuit is to be considered merely'for simplified illustration in accordance with established practice. Furthermore, various other types of tube may as well be used as long as circuit constants are suitably proportioned as required by the tube characteristics.

Describing the operation of the system, let it be assumed that carrier-frequency energy is supplied from the oscillator and amplifier to the grids 3, 3 and 4 of the tubes l, I' and 2, and at this particular instant no audio-frequency voltage is delivered by the modulation-frequency a negative potential with respect to ground will be inactive and the output thereof, of course, zero. -In the next half cycle of modulation-frequency, the polarity of the secondary winding 24 reverses and now the tube which formerly was active becomes inactive, and vice versa. Alternately, the tube I and thereafter tube I will be supplied with a proper polarity 'of potential to act as an amplifier for the constant carrierfrequency at a rate of the modulation-frequency supplied, each delivering, at one-half cycle a r varying amplitude radio-frequency to the load amplifier, that is, the microphone 21 is not en- 0 circuit comprising the primary winding 35 and consequently the antenna 38. Considering the action of the three tubes, We have a carrier-output from one tube, that is, tube 2, and alternate radio-frequency outputs varying in amplitude at the modulating-frequency as determined by'the polarity of the winding 24 of the transformer 25.

Now, if the output circuits are so arranged that the output of one tube is in phase at radiofrequency with the carrier and the output from the other is 180 out of phase, which is accomplished by simply reversing the connections of the secondary windings 33 or 33', the conditions are correct for reproduction of the modulation signal in the form of amodulated radio-frequency output wave. It can beseen that under these conditions one tube adds its output to the carrier during one modulation-frequency halfe cycle and the other, subtracts from the carrier during the next half-cycle." The action illustrating the process of modulation can be seen by referring to Fig. 2. Q

Curve A represents the anode potential of tube I, for example,,and curve B that of tube I, the potentials being 180 out of phase. .Curve 0 represents the carrier'output of tubes I and I. Curve D represents the output voltage of tube 2. The resultant dual output'voltage into the load circuit isshown by the curve E which illustrates modulation of the carrier-frequency:en-' ergy. obtained in accordance with the'sys'tem herein described.

Curve F represents the conditions existing when the output circuits of tubes I and I are improperly, phased, that is, when there is zero phase difference. Under such conditions the'sy'stem will'not'; operate in the manner intended, and distortion takes place. The modulation then is not the original sine wave, but is composedof double'frequency, half-sine pulses'which do notresemble the original audio signal;

We claim as'our;invention': j 1. In a system; of 'radio transmission, a car-- rier-frequency: source; i amodulation circuit,- comprising, transmission channels, each containing a vacuum three "separate carrier-"frequency tube having anode, cathode and at least one control electrode, a common connection between said cathodes, a circuit interconnecting said control electrodes and said source, whereby the input of all of said tubes between control electrodes and cathodes is simultaneously energized from said source; an output circuit for each of said tubes between anodes and cathodes including an inductance tunable to said carrier frequency, a power supply for anode potential connected between cathode and anode of one of said tubes, whereby one of said channels is operable to supply a carrier of constant amplitude, an alternating current source of potential for modulating said carrier, a divided output circuit therefor, circuit means for terminating said divided output circuit in opposite phase relation between cathodes and anodes of said tubes in said other channels, coupling means for combining the output of one of said channels in additive and the output of another of said channels in subtractive relation with respect to said channel of constant amplitude and means for eifectively combining the output of all of said channels.

2. In a modulation system for high frequency energy, a plurality of vacuum tubes each having anode, cathode and at least a control electrode, means for simultaneously energizing the input circuit of each of said tubes between control electrode and cathode thereof by a voltage of constant frequency, an output circuit for each of said tubes between anode and cathode thereof, each containing an impedance resonant to said frequency, a source supplying direct current operating potential connected between cathode and anode of one of said tubes, and a second source of operating potential connected between anodes and cathodes of the others of said tubes, said second source being of an alternating character and of a frequency desired to modulate said high frequency energy, and circuit means interconnecting said output circuits in additive and subtractive phase relation respectively, between one of said tubes and said other tubes.

3. The method of modulation which comprises generating a carrier-frequency, amplifying said carrier in three separate stages by simultaneously applying said carrier in phase to the input circuits of vacuum tube amplifiers, maintaining the carrier-frequency output of one of said. amplifiers substantially constant, alternately varying in amplitude the carrier outputof two of said amplifiers at a modulation-frequency rate and in opposite phase, combining the outputs of said amplifiers in such phase relation as to obtain increased carrier output at one-half cycle of said modulation-frequency and a decreased output at another half cycle thereof.

4. The method of modulation which comprises generating a carrier-frequency, amplifying said carrier in three separate stages by simultaneously applying said carrier in phase to the input circuits of vacuum tube amplifiers, maintaining the carrier-frequency output of one of said amplifiers substantially constant, alternately varying in amplitude the carrier output of two of said amplifiers at a modulation-frequency rate and in opposite phase, combining the outputs of said amplifiers in such phase relation as to obtain increased carrier output at one-half cycle of said modulation-frequency and a decreased output at another half cycle thereof by amount corresponding to the carrier output of said alternately varied amplifiers.

5. The method of modulating a carrier-frequency of constant amplitude which comprises impressing said carrier-frequency cophasely upon three transmission channels, controlling the transmission of said carrier in two of said channels in opposition at the modulation frequency rate, transmitting said carrier in said third channel at a constant amplitude, combining the output of all of said channels additively as to one of said channels and said third channel and subtractively as to another of said channels and said third channel.

HENRY N. KOZANOWSKI. ILIA E. MOUROMTSEFF. 

